Original Article With Video Illustration

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1 Original Article With Video Illustration Correlation Between the 2-Dimensional Width and the 3-Dimensional Volume: A Cadaveric Study Carola F. Van Eck, M.D., Cesar A. Q. Martins, M.D., Sebastian Kopf, M.D., Pisit Lertwanich, M.D., Freddie H. Fu, M.D., D.Sc., and Scott Tashman, Ph.D. Purpose: The purpose of this study was to compare the size of the entrance of the notch, as measured arthroscopically (2-dimensionally), with the volume of the notch as measured by use of computed tomography (CT) (3-dimensionally). Methods: For 20 cadaveric knees, the dimensions of the notch entrance were measured arthroscopically, and the notch volume was measured by use of CT. The correlation between the size of the notch entrance and the notch volume was calculated. Intraobserver reliability and interobserver reliability of the arthroscopic and CT measurements were tested. Results: The Pearson correlation coefficients between CT-assessed notch volume and arthroscopically assessed notch height and width at the bottom, middle, and top of the notch were 0.603, 0.506, 0.551, and 0.642, respectively. The intraobserver reliability and interobserver reliability of the arthroscopic measurements were above and 0.819, respectively, and and 0.975, respectively, for the CT measurements. Conclusions: There were only moderate correlations between arthroscopic notch measurements and notch volume. Both the arthroscopic and CT measurements proved highly reliable. Clinical Relevance: The moderate correlation between 2-dimensional and 3-dimensional notch measurements warrants caution concerning the use of either measurement for assessing risk for anterior cruciate ligament injury or as justification for notchplasty until studies between the relation of 3-dimensional notch volume and anterior cruciate ligament injury are conducted. Norwood and Cross 1 showed anatomic relations between the anterior cruciate ligament (ACL) and the femoral notch. They showed that the ACL was tight From the Department of Orthopaedic Surgery, University of Pittsburgh Medical Center (C.F.v.E., C.A.Q.M., S.K., P.L., F.H.F., S.T.), Pittsburgh, Pennsylvania, U.S.A.; and Orthopaedic Research Centre Amsterdam, Academic Medical Centre (C.F.v.E.), Amsterdam, The Netherlands. The University of Pittsburgh receives a research grant from Smith & Nephew to support research related to anterior cruciate ligament reconstruction. The authors report no conflict of interest. Received December 10, 2009; accepted June 28, Address correspondence and reprint requests to Scott Tashman, Ph.D., Department of Orthopaedic Surgery, Orthopaedic Research Laboratories, University of Pittsburgh, 3820 S Water St, Pittsburgh, PA 15203, U.S.A. tashman@pitt.edu 2011 by the Arthroscopy Association of North America /9738/$36.00 doi: /j.arthro Note: To access the video accompanying this report, visit the February issue of Arthroscopy at against the intercondylar shelf in extension, which concentrated forces in the midsection of the ligament. Chondrocytes have been identified on the ACL surface around the aforementioned area, 2 which may help to protect the ACL from pressure arising from rubbing of the ACL on the intercondylar notch. From the results of these 2 studies, it was hypothesized that a smaller notch increases ACL-notch contact, which might increase the risk of ACL injury. Some studies confirm this relation between notch size and the incidence of ACL rupture, 3-9 whereas others have found no relation. 10,11 Typically, 2-dimensional (2D) notch measurements have included the intercondylar notch width, 12,13 notch area, transverse and sagittal notch angles, 14 bicondylar width, and notch width index. 15 These measurements were performed either in cadaveric studies, 13 on plain radiographs, 4,6-8,15 arthroscopically in vivo, 12 or on computed tomography (CT) scans 3,5 or magnetic resonance images. 11,14 To our knowledge, only 1 study has measured the notch 3- dimensionally, and this study only compared the notch Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 27, No 2 (February), 2011: pp

2 208 C. F. VAN ECK ET AL. size between men and women. 16 It is unknown whether 2D or 3-dimensional (3D) measurements of intercondylar notch have a better relation with the incidence of ACL rupture. During ACL surgery, most attention is paid to the entrance of the notch, 12,17 because when the notch entrance is too small, notchplasty is performed to prevent impingement. A small notch may also be a contraindication for double-bundle reconstruction, because of lack of space It is currently not known whether the size of the entrance of the notch corresponds with the volume of the notch (i.e., persons with a narrow notch entrance may have a large notch volume). The aim of this study is to compare the size of FIGURE 1. Axial CT images of a right knee. The borders of the notch are outlined with green. The box displays the area of this outline. (A) First included slice from proximal to distal: first image with both femoral condyles visible. (B) Posterior border of the notch: line between the 2 points on the inside of the femoral condyles with a sudden change of slope. (C) Last included slice: most distal image with continuity between the medial and lateral femoral condyles. (D) The notch volume can be calculated by summation of the area of all images within the defined femoral notch and multiplication by the slice thickness. It should be noted that this figure is a simplification of the method with only 3 images; an average notch will have between 30 and 100 images. The boxes display the measured area of the green outline of the notch in cm3.

3 NOTCH ENTRANCE AND VOLUME MEASUREMENT 209 the notch entrance, as measured arthroscopically (2- dimensionally), with the notch volume, as measured on CT (3-dimensionally). Our hypothesis was that the size of the notch entrance does not correlate well with the volume of the notch. METHODS This was a controlled laboratory study. Twenty cadaveric knees were used. Specimens with signs of osteoarthritic changes (Outerbridge grade 3 or higher as evaluated on CT scan or arthroscopically) were excluded. First, the knees underwent CT imaging (slice thickness, mm). The notch volume was measured by a method based on a technique developed for magnetic resonance imaging by Charlton et al. 16 Manual digital tracings of the femoral intercondylar notch perimeter were performed on the axial CT images by use of Osirix software (Osirix, Geneva, Switzerland). FIGURE 2. Arthroscopic images of femoral notch of a right knee: anteromedial portal view with knee in 90 of flexion. The excursion of the guidewire from the tip of the needle is used to measure the dimensions of the notch entrance. (A) The height is measured along the lateral wall of the notch. (B) The width at the bottom of the notch is measured. (C and D) The widths at the middle and top of the notch are measured, based on the measured height. (ACL, anterior cruciate ligament; LFC, lateral femoral condylar; PCL, posterior cruciate ligament.)

4 210 C. F. VAN ECK ET AL. TABLE 1. Intraclass and Interclass Correlation Coefficient of Measurements on CT Scan and Arthroscopically Observer 1 Observer 2 SEM volume (observer 1) ( ) ( ) 0.47 cm 3 height (observer 1) ( ) ( ) 0.73 mm width bottom (observer 1) ( ) ( ) 0.84 mm width middle (observer 1) ( ) ( ) 0.58 mm width top (observer 1) ( ) ( ) 2.0 mm NOTE. The 95% confidence intervals are given in parentheses. Abbreviation: SEM, standard error of measurement. volume was calculated by summation of the measured areas of the notch outlines from each slice and multiplication by the slice thickness. The boundaries of the notch were defined based on anatomic landmarks. The most proximal slice was the first image (from proximal to distal) with both femoral condyles visible (verified on the sagittal view as being the roof of the notch). The most distal slice was the last image with continuity between the medial and lateral femoral condyles. The posterior border of the notch on each image was defined by a line drawn between the 2 points on the inside of the femoral condyles with a sudden change of slope (Fig 1). Two independent observers performed all measurements. One of the observers performed each measurement twice, at least 2 weeks apart. The scans were analyzed in random order to eliminate possible training effects. After the CT scanning was performed, the cadaveric specimens underwent knee arthroscopy. During this procedure, measurements were taken from the notch entrance by use of a newly developed measuring system. This system uses a flexible needle with a guidewire, which was introduced from superior and medial into the knee joint. By measuring excursion of the guidewire from the needle, the desired notch entrance measurements could be obtained, as shown in Fig 2 and Video 1 (available at The following dimensions of the notch were measured: notch height and width at the bottom, middle (one-third of notch height), and top (two-thirds of notch height) of the notch. One-third and two-thirds of the notch height were calculated by use of the height measurement. Two independent observers performed all measurements. One of the observers performed each measurement twice, at least 2 weeks apart. Statistical analysis To establish the reliability between 2 independent measurements by the same observer for the notch volume measurements on CT scan and the arthroscopic measurements of the notch entrance, the intraclass correlation coefficient was determined. To establish the reliability between the 2 observers, the interclass correlation coefficient was determined. The first CT and arthroscopic measurements of the first observer were used for statistical comparisons between the 2 methods. The mean, range, standard deviations, and standard error of measurements were calculated for all measurements. Correlations between the 3D notch volume (from CT) and the 2D arthroscopic measurements (notch height and notch width at the bottom, middle, and top of the notch) were calculated by use of the Pearson correlation coefficient (P.05 for significance). An independent t test was used to determine whether there was a difference in notch volume, notch height, and notch width between men and women. RESULTS Reliability for notch volume and arthroscopic measurements was high, with mean intraclass and interclass correlation coefficients greater than 0.97 and 0.82, respectively (Table 1). The mean age of the specimens was 58 years, as shown in Table 2 (along with other demographic data). The measurements of notch height, width, and volume are shown in Table 3. Moderate (but statistically significant) correlations were found between CT- TABLE 2. Age (yr) Demographic Data of 20 Included Cadaveric Knees Height (cm) Weight (kg) M/F L/R Mean SD Min Max Ratio 10/10 10/10 Abbreviations: Min, minimal value; Max, maximal value.

5 NOTCH ENTRANCE AND VOLUME MEASUREMENT 211 TABLE 3. Measurements Taken From 20 Cadaveric Specimens Mean SD Min Max TABLE 5. Difference in Size Between Men and Women Mean volume (cm 3 ) height (mm) width bottom (mm) width middle (mm) width top (mm) Abbreviations: Min, minimal value; Max, maximal value. based notch volume and all arthroscopic measurements, with correlation coefficients ranging from 0.51 to 0.64 (Table 4). There was a significant difference in notch size between men and women. Men had a larger notch volume (P.004), notch height (P.003), and notch width at the bottom (P.015), middle (P.013), and top (P.005) of the notch (Table 5). DISCUSSION The aim of this study was to compare the size of the entrance of the notch, as measured arthroscopically (2- dimensionally), with the volume of the notch as measured on CT scan (3-dimensionally). We hypothesized that the size of the notch entrance would not correlate well with the volume of the notch. Indeed, we found only moderate correlations between the dimensions of the notch entrance and the notch volume. These results suggest that arthroscopic measurements of the entrance of the femoral intercondylar notch provide a relatively poor approximation of actual notch volume. Many studies have suggested a relation between notch size and incidence of ACL rupture, 3,4 but none of these studies is based on 3D measurements. A recent study by Anderson et al. 22 showed that the most often used measurement, the notch width index, is not accurate with 2 of the 3 frequently used radiographic techniques. Two-dimensional measurements are also Pearson Correlation Coefficient Between Height/Width and Volume, Including Significance Level TABLE 4. Height Width Bottom Width Middle Width Top volume P value.005*.023*.012*.002* *Significant correlation. Men Women P Value volume (cm 3 ) * height (mm) * width bottom (mm) * width middle (mm) * width top (mm) * *Significant difference. sensitive to rotation and angulation. 6 Our 3D CT measurements avoided these errors and proved to be highly reliable. The 2D arthroscopic measurements of the notch entrance were also found to be reliable in this study. A smaller notch often is seen as an indication to perform notchplasty to avoid impingement. 23 This study shows that a small notch entrance does not necessarily indicate small notch volume. However, we do recognize that the 2D size of the notch entrance is very important during ACL surgery. A small notch entrance can make double-bundle reconstruction challenging, because it can be hard to reach the femoral anteromedial insertion site without damaging the medial femoral condyle with the drill. Thus, when the notch entrance is smaller than 12 mm in width at the bottom (data from an ongoing study), we recommend anatomic single-bundle reconstruction without notchplasty, to preserve as much of the patient s native anatomy as possible. 18,19 In addition, at this point, it is still unknown whether 3D measurements will provide a better correlation with the risk for ACL rupture than 2D measurements. Just as the ACL insertion site has various shapes and sizes, 24 the notch shape and size also vary among patients. 25 The results of this study confirm wide variability for both notch volume and notch entrance dimensions. We did find a significantly higher notch volume, height, and width at the bottom for men compared with women. This has also been shown in other studies but may be largely because of the difference in height and weight. 16 Limitations of this study include the use of cadaveric specimens that are considerably older than the mean age of patients with ACL ruptures, as well as a relatively small sample size. However, there is no reason to expect that the relation between notch entrance dimensions and notch volume would change

6 212 C. F. VAN ECK ET AL. significantly with age, as long as no arthritic changes such as osteophytes are present. CONCLUSIONS This study showed only moderate correlations between arthroscopic 2D notch entrance measurements and 3D notch volume. Both the arthroscopic and CT measurements proved highly reliable. REFERENCES 1. Norwood LA Jr, Cross MJ. The intercondylar shelf and the anterior cruciate ligament. Am J Sports Med 1977;5: Petersen W, Tillmann B. Structure and vascularization of the cruciate ligaments of the human knee joint. Anat Embryol (Berl) 1999;200: Anderson AF, Lipscomb AB, Liudahl KJ, Addlestone RB. Analysis of the intercondylar notch by computed tomography. Am J Sports Med 1987;15: Houseworth SW, Mauro VJ, Mellon BA, Kieffer DA. The intercondylar notch in acute tears of the anterior cruciate ligament: A computer graphics study. Am J Sports Med 1987; 15: Hernigou P, Garabedian JM. Intercondylar notch width and the risk for anterior cruciate ligament rupture in the osteoarthritic knee: Evaluation by plain radiography and CT scan. Knee 2002;9: Ireland ML, Ballantyne BT, Little K, McClay IS. A radiographic analysis of the relationship between the size and shape of the intercondylar notch and anterior cruciate ligament injury. Knee Surg Sports Traumatol Arthrosc 2001;9: LaPrade RF, Burnett QM II. Femoral intercondylar notch stenosis and correlation to anterior cruciate ligament injuries. A prospective study. Am J Sports Med 1994;22: ; discussion Lund-Hanssen H, Gannon J, Engebretsen L, Holen KJ, Anda S, Vatten L. Intercondylar notch width and the risk for anterior cruciate ligament rupture. A case-control study in 46 female handball players. Acta Orthop Scand 1994;65: Souryal TO, Freeman TR. Intercondylar notch size and anterior cruciate ligament injuries in athletes. A prospective study. Am J Sports Med 1993;21: Lombardo S, Sethi PM, Starkey C. Intercondylar notch stenosis is not a risk factor for anterior cruciate ligament tears in professional male basketball players: An 11-year prospective study. Am J Sports Med 2005;33: Herzog RJ, Silliman JF, Hutton K, Rodkey WG, Steadman JR. Measurements of the intercondylar notch by plain film radiography and magnetic resonance imaging. Am J Sports Med 1994;22: Shelbourne KD, Facibene WA, Hunt JJ. Radiographic and intraoperative intercondylar notch width measurements in men and women with unilateral and bilateral anterior cruciate ligament tears. Knee Surg Sports Traumatol Arthrosc 1997;5: Stijak L, Radonjic V, Nikolic V, Blagojevic Z, Aksic M, Filipovic B. Correlation between the morphometric parameters of the anterior cruciate ligament and the intercondylar width: Gender and age differences. Knee Surg Sports Traumatol Arthrosc 2009;17: Cha JH, Lee SH, Shin MJ, Choi BK, Bin SI. Relationship between mucoid hypertrophy of the anterior cruciate ligament (ACL) and morphologic change of the intercondylar notch: MRI and arthroscopy correlation. Skeletal Radiol 2008;37: Souryal TO, Moore HA, Evans JP. Bilaterality in anterior cruciate ligament injuries: Associated intercondylar notch stenosis. Am J Sports Med 1988;16: Charlton WP, St John TA, Ciccotti MG, Harrison N, Schweitzer M. Differences in femoral notch anatomy between men and women: A magnetic resonance imaging study. Am J Sports Med 2002;30: Shelbourne KD, Davis TJ, Klootwyk TE. The relationship between intercondylar notch width of the femur and the incidence of anterior cruciate ligament tears. A prospective study. Am J Sports Med 1998;26: Martins CAQ, Kropf EJ, Shen W, van Eck CF, Fu FH. The concept of anatomic anterior cruciate ligament reconstruction. Oper Tech Sports Med 2008;16: Shen W, Forsythe B, Ingham SM, Honkamp NJ, Fu FH. Application of the anatomic double-bundle reconstruction concept to revision and augmentation anterior cruciate ligament surgeries. J Bone Joint Surg Am 2008;90:20-34 (Suppl 4). 20. Schreiber VM, van Eck CF, Fu FH. Anatomic double-bundle ACL reconstruction. Sports Med Arthrosc 2010;18: van Eck CF, Lesniak BP, Schreiber VM, Fu FH. Anatomic single- and double-bundle anterior cruciate ligament reconstruction flowchart. Arthroscopy 2010;26: Anderson AF, Anderson CN, Gorman TM, Cross MB, Spindler KP. Radiographic measurements of the intercondylar notch: Are they accurate? Arthroscopy 2007;23: , 268.e1-268.e Good L, Odensten M, Gillquist J. Intercondylar notch measurements with special reference to anterior cruciate ligament surgery. Clin Orthop Relat Res 1991: Kopf S, Musahl V, Tashman S, Szczodry M, Shen W, Fu FH. A systematic review of the femoral origin and tibial insertion morphology of the ACL. Knee Surg Sports Traumatol Arthrosc 2009;17: Farrow LD, Chen MR, Cooperman DR, Victoroff BN, Goodfellow DB. Morphology of the femoral intercondylar notch. J Bone Joint Surg Am 2007;89: